Fair Value Measurements (Fair Value of Assets and Liabilities Measured on Recurring Basis) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Sep. 30, 2019	Dec. 31, 2017
Proceeds from Sale of Debt Securities, Available-for-sale	$ 0.5
AvailableForSaleSecuritiesGross Realized Gains Losses Sale Proceeds Net of Tax	0.4
Fair Value, Inputs, Level 1 [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.4	$ 0.2	$ 0.3
Total Derivative Assets	0.0	0.0	0.0
Total Assets	0.4	0.2	0.3
Debt Instrument, Fair Value Disclosure			0.0
Total Liabilities	0.0	0.0	0.0
Fair Value, Inputs, Level 1 [Member] \| Subsidiaries [Member] \| Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.0	0.0
Fair Value, Inputs, Level 2 [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	7.7	8.2	8.8
Total Derivative Assets	1.5	0.2	1.5
Total Assets	9.2	8.4	10.3
Debt Instrument, Fair Value Disclosure			640.6
Total Liabilities	576.1	613.4	640.6
Fair Value, Inputs, Level 2 [Member] \| Subsidiaries [Member] \| Interest Rate Contract [Member]
Total Derivative Assets	1.5	0.2	1.5
Fair Value, Inputs, Level 3 [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Total Derivative Assets	0.0	0.0
Total Assets	0.0	0.0	0.0
Debt Instrument, Fair Value Disclosure			17.8
Total Liabilities	17.7	17.6	17.8
Fair Value, Inputs, Level 3 [Member] \| Subsidiaries [Member] \| Interest Rate Contract [Member]
Total Derivative Assets	0.0	0.0
Equity Securities [Member] \| Fair Value, Inputs, Level 1 [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.0	0.0
Equity Securities [Member] \| Fair Value, Inputs, Level 2 [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	3.5	3.9	4.2
Equity Securities [Member] \| Fair Value, Inputs, Level 3 [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Debt Securities [Member] \| Fair Value, Inputs, Level 1 [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.0	0.0
Debt Securities [Member] \| Fair Value, Inputs, Level 2 [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	4.0	4.1	4.3
Debt Securities [Member] \| Fair Value, Inputs, Level 3 [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Money Market Funds [Member] \| Fair Value, Inputs, Level 1 [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.4	0.2	0.3
Money Market Funds [Member] \| Fair Value, Inputs, Level 2 [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.0	0.0
Money Market Funds [Member] \| Fair Value, Inputs, Level 3 [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Hedge Funds [Member] \| Fair Value, Inputs, Level 1 [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.0	0.0
Hedge Funds [Member] \| Fair Value, Inputs, Level 2 [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.1	0.1	0.2
Hedge Funds [Member] \| Fair Value, Inputs, Level 3 [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Tangible Assets [Member] \| Fair Value, Inputs, Level 1 [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.0	0.0
Tangible Assets [Member] \| Fair Value, Inputs, Level 2 [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.1	0.1	0.1
Tangible Assets [Member] \| Fair Value, Inputs, Level 3 [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.0	0.0
Debt [Member] \| Fair Value, Inputs, Level 1 [Member] \| Subsidiaries [Member]
Debt Instrument, Fair Value Disclosure	0.0	0.0
Debt [Member] \| Fair Value, Inputs, Level 2 [Member] \| Subsidiaries [Member]
Debt Instrument, Fair Value Disclosure	576.1	613.4
Debt [Member] \| Fair Value, Inputs, Level 3 [Member] \| Subsidiaries [Member]
Debt Instrument, Fair Value Disclosure	17.7	17.6
Fair Value [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	8.1	8.4	9.1
Total Derivative Assets	1.5	0.2	1.5
Total Assets	9.6	8.6	10.6
Total Liabilities	593.8	631.0	658.4
Fair Value [Member] \| Subsidiaries [Member] \| Interest Rate Contract [Member]
Total Derivative Assets	1.5	0.2	1.5
Fair Value [Member] \| Equity Securities [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	3.5	3.9	4.2
Fair Value [Member] \| Debt Securities [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	4.0	4.1	4.3
Fair Value [Member] \| Money Market Funds [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.4	0.2	0.3
Fair Value [Member] \| Hedge Funds [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.1	0.1	0.2
Fair Value [Member] \| Tangible Assets [Member] \| Subsidiaries [Member]
Total Master Trust Assets, Fair Value	0.1	0.1	0.1
Fair Value [Member] \| Debt [Member] \| Subsidiaries [Member]
Debt Instrument, Fair Value Disclosure	593.8	631.0	658.4
Long-term Debt, Fair Value	$ 593.8	$ 631.0	$ 658.4

Realized gains and losses from sale of available for sale securities, net of tax.

No definition available.

Name:	dpl_AvailableforsalesecuritiesgrossRealizedGainsLossesSaleProceedsNetOfTax
Namespace Prefix:	dpl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_AssetsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611

+ Details

Name:	us-gaap_DebtInstrumentFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

Name:	us-gaap_DerivativeAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_InvestmentsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of financial and nonfinancial obligations.

+ References

+ Details

Name:	us-gaap_LiabilitiesFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

+ Details

Name:	us-gaap_LongTermDebtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of cash inflow from sale of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26853-111562

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3151-108585

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3179-108585

+ Details

Name:	us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ConsolidatedEntitiesAxis=srt_SubsidiariesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_DebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_MoneyMarketFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_HedgeFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=dpl_TangibleAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_DebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EstimateOfFairValueFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: